[RFC PATCH v2 24/41] arm64/sve: Discard SVE state on system call

[Mark Rutland]

On Wed, Mar 22, 2017 at 02:50:54PM +0000, Dave Martin wrote:
> The base procedure call standard for the Scalable Vector Extension
> defines all of the SVE programmer's model state (Z0-31, P0-15, FFR)
> as caller-save, except for that subset of the state that aliases
> FPSIMD state.
> 
> System calls from userspace will almost always be made through C
> library wrappers -- as a consequence of the PCS there will thus
> rarely if ever be any live SVE state at syscall entry in practice.
> 
> This gives us an opportinity to make SVE explicitly caller-save
> around SVC and so stop carrying around the SVE state for tasks that
> use SVE only occasionally (say, by calling a library).
> 
> Note that FPSIMD state will still be preserved around SVC.
> 
> As a crude heuristic to avoid pathological cases where a thread
> that uses SVE frequently has to fault back into the kernel again to
> re-enable SVE after a syscall, we switch the thread back to
> FPSIMD-only context tracking only if the context is actually
> switched out before returning to userspace.
> 
> Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> ---
>  arch/arm64/kernel/fpsimd.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 5fb5585..8c18384 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -250,6 +250,23 @@ static void task_fpsimd_save(struct task_struct *task)
>  	BUG_ON(task != current);
>  
>  	if (IS_ENABLED(CONFIG_ARM64_SVE) &&
> +	    task_pt_regs(task)->syscallno != ~0UL &&
> +	    test_tsk_thread_flag(task, TIF_SVE)) {
> +		unsigned long tmp;
> +
> +		clear_tsk_thread_flag(task, TIF_SVE);
> +
> +		/* Trap if the task tries to use SVE again: */
> +		asm volatile (
> +			"mrs	%[tmp], cpacr_el1\n\t"
> +			"bic	%[tmp], %[tmp], %[mask]\n\t"
> +			"msr	cpacr_el1, %[tmp]"
> +			: [tmp] "=r" (tmp)
> +			: [mask] "i" (CPACR_EL1_ZEN_EL0EN)
> +		);

Given we're poking this bit in a few places, I think it would make more
sense to add enable/disable helpers. Those can also subsume the lazy
writeback used for the context switch, e.g.

static inline void sve_el0_enable(void)
}
	unsigned long cpacr = read_sysreg(cpacr_el1);
	if ((cpacr & CPACR_EL1_ZEN_EL0EN)
		return;

	cpacr |= CPACR_EL1_ZEN_EL0EN;
	write_sysreg(cpacr, cpacr_el1);
}

static inline void sve_el0_disable(void)
{
	unsigned long cpacr = read_sysreg(cpacr_el1);
	if (!(cpacr & CPACR_EL1_ZEN_EL0EN)
		return;

	cpacr &= ~CPACR_EL1_ZEN_EL0EN;
	write_sysreg(cpacr, cpacr_el1);
}

Thanks,
Mark.